Surprise: Radioactive Mercury Decays Into Uneven Chunks

More than seven decades after German chemists discovered nuclear fission — the splitting of an atom that is harnessed by nuclear energy and nuclear weapons — scientists still can’t describe the process in detail. A paper to appear in Physical Review Letters underscores that knowledge gap with the report of a totally unexpected type of fission in the element mercury. Instead of splitting into two equal-mass chunks as theory predicts, this bit of mercury split into uneven chunks, one lighter and one heavier than expected.

Asymmetric fission, which results in daughter fragments with different masses, has been seen before. But these earlier examples all could be easily explained. Isotopes of uranium, for instance, like to fission into one large chunk of tin-132 along with a smaller chunk. Like apartment dwellers filling each apartment in a complex, the 50 protons and 82 neutrons of tin-132 completely fill shells, or energy levels, within the nucleus and hence make it extremely stable.

In the new experiments, researchers thought the isotope mercury-180 would split equally into blobs of zirconium-90, which has 40 protons and 50 neutrons that stably fill the shells in the nucleus. “Zirconium-90 plus zirconium-90 makes mercury-180,” says Witold Nazarewicz, a theoretical physicist at the University of Tennessee in Knoxville and the Oak Ridge National Laboratory who was not involved in the work.

Yet that’s not what the scientists saw in their experiments at the ISOLDE radioactive beam facility at CERN, Europe’s particle physics laboratory near Geneva. The researchers, led by Andrei Andreyev of the University of the West of Scotland in Paisley, instead saw the mercury-180 fission unevenly into ruthenium-100 and krypton-80 — isotopes that don’t have completely filled shells the way zirconium-90 does.

Not only were the products of mercury-180 fission asymmetric, but it’s the first time researchers have seen asymmetric fission and not been able to explain it by the filled-shells theory. “It was a big surprise,” says team member Piet Van Duppen, a nuclear physicist at the Catholic University of Leuven in Belgium. “This is a totally new form of asymmetric fission.”

Puzzled, the scientists analyzed the energy it takes mercury-180 to fission. The most energy-efficient way turns out to be to split into ruthenium-100 and krypton-80 rather than equal parts of zirconium-90, Van Duppen says.

Other isotopes in the same part of the periodic table might also show the same uneven split, he says. The team has already tested a second isotope of mercury and seen asymmetric fission there.

Probing fission throughout the periodic table will get easier with a new generation of radioactive beam facilities coming online in the next decade, says Van Duppen. These include the Facility for Rare Isotope Beams at Michigan State University in East Lansing and the Facility for Antiproton and Ion Research at the GSI research center in Darmstadt, Germany.

“What we have here,” he adds, “is a new experimental tool to really verify our understanding of the atomic nucleus.”